20225: Add subdirectory selection support to directory array picker

20225: Add subdirectory selection support to directory array picker

Adds cascade flag to tree picker to disable recursive directory selection

Reworks initProjectsTreePicker to support initializing multiple selections.
Loads each selection's ancestor tree in parallel, combines updates to same tree
pickers before inserting the ancestor tree, then loads necessary collections in
parallel

Changes checkbox visibility logic to show collection selection checkbox even
when collection is not loaded/expanded when cascade mode is off - since the
selection won't cascade there is no need to require the collection to be opened

Arvados-DCO-1.1-Signed-off-by: Stephen Smith <stephen@curii.com>